The present invention relates to the measurement of nuclear magnetic resonance (NMR) and, in particular, to the NMR measurement of polycrystalline and/or amorphous solids.
Since its discovery in 1946, Nuclear Magnetic Resonance (NMR) has become a powerful analytical tool in studies of various liquid and solid materials. It is non-invasive and gives results which can be readily interpreted. The measured chemical shifts can be immediately associated with model structural units of the substance under study. In addition, relaxation rates provide information about molecular motions. Characteristic resonance frequencies of magnetic nuclei lie in a convenient radio-frequency (RF) range and can be detected with well-known electronic techniques.
An NMR experiment may be described as follows. Nuclei may be regarded as bar magnets that, because of their charge and nuclear spin, have associated magnetic moments. These magnetic moments are randomly oriented in the absence of external forces. When placed in an external magnetic field, these nuclei have discrete spin states. A quantized amount of energy is associated with each such spin state. The energy of each such state depends on the nuclear magnetic moment of the nuclei being studied and magnetic field in the vicinity of the nucleus in question. This magnetic field is the superposition of the external magnetic field and the magnetic fields generated by nearby electrons and nuclei. Hence, measurements of the energy levels associated with each of the spin states can provide information about the environment of the nuclei being studied.
An NMR measurement is made by determining the energy difference between nuclear spin states. In order to accomplish this, a sample of the material in question is placed in an external magnetic field and excited by applying a second, oscillating magnetic field in a direction perpendicular to the first steady field. This is accomplished by applying oscillating RF energy across a coil positioned perpendicular to the direction of the external magnetic field. A second magnetic field is created by a pulsing current in this coil. This second field causes transitions between nuclear spin states whose energies are determined by the first field. The energy absorbed by the nuclei during such an excitation or emitted thereby after such an excitation provides information on the differences in energy between the various spin states.
The accuracy of NMR measurements depends upon the physical form of the sample being studied. Highly accurate chemical shift determinations and separation of NMR lines are possible for liquid samples due to the random tumbling and rapid reorientation of sample molecules in solution. This rapid reorientation effectively causes the surroundings of the resonating nuclei to appear isotropic on the time scale of the NMR experiment.
If polycrystalline, powdery, glassy solids, or the like are studied, however, the observable lines are broadened due to different orientations of particles with respect to the static magnetic field. Anisotropic line broadening has traditionally rendered high-resolution work impossible with this type of sample.
Various methods have been employed in order to reduce the amount of line broadening observed for solid samples. For example, techniques in which crystalline materials are oriented in a particular direction with respect to the external magnetic field are known to the prior art. Unfortunately, such methods are impractical in many cases, since many solids cannot be obtained in single crystalline form nor oriented in a uniform direction throughout the material. In fact, many samples for which NMR spectra are desired exist only as powders or amorphous solids.
In the prior art, line broadening difficulties can be partially overcome by using magic angle spinning (MAS). Using this technique, the sample is rotated rapidly at an angle of 54.7 degrees with respect to the external magnetic field, i.e., the magic angle. For reasons that will be discussed in more detail below, this spinning removes so-called first order line broadening caused by such factors as chemical shift anisotropy, secular dipolar interactions, and first order quadrupole interactions. As a result, line widths on the order of 100 Hz are typically observed for non-quadrupole nuclei.
Although these line widths are a significant improvement over those obtained without MAS, they are still far broader than those obtained with liquid samples. Typically, line widths of 0.2 Hz are observed for liquids.
In the case of quadrupole nuclei, the line broadening is even worse. Line widths of quadrupole nuclei are determined primarily by second order quadrupole interactions, and are on the order of several KHz or more for light nuclei in strong magnetic fields. Magic angle spinning does not completely correct for this line broadening.
In a co-pending application (U.S. Ser. No. 227,729) an apparatus and method for reducing the line broadening from such second order interactions is disclosed. The apparatus in question re-orients the sample during the measurements such that the average values of certain generalized spherical harmonic functions is zero. This improved method and apparatus requires that the sample be moved in a time which is small compared to the NMR relaxation time for the sample being measured. An apparatus for providing this rapid reorientation of the sample is expensive to manufacture.
Furthermore, it is not always possible to provide this type of rapid reorientation. For example, if superconducting samples are to be studied, the samples must be maintained at very low temperatures. Providing rapid sample reorientation at such temperatures is difficult.
Accordingly, it is an object of the present invention to provide an improved apparatus for measuring nuclear magnetic properties of solids.
It is a further object of the present invention to provide an NMR apparatus with improved resolution for structural determinations of powdered or amorphous or otherwise orientationally disordered solid samples.
It is another object of the present invention to provide an apparatus and method which allows NMR spectra to be measured even when the sample cannot be moved in a time which is short compared to the NMR relaxation time.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.